Will it be possible for the community to build mods for this game? I am thinking about an alternate weather chart and alternate Oil rules.

Not really. There are a lot of optional rules (80) and you can edit the numbers on the units, even adding new units if you like. But beyond that not much is purely data driven.

For instance, the cost in range for an air unit to enter/leave a sea area section box is hard-coded. The probabilities for US Entry Actions/Options drawing a chit are also hard coded. All the combat tables are too.

My job is to transfer WIF to the computer (MWIF), which is a big enough task in and of itself. Creating a WIF modification kit was never part of my contract. Where it was trivial to make something available to the players to make changes, I did make some things available. But the rules and the code are tightly interwoven, primarily because the rules are so interconnected.

If all you have in mind is changing the oil cost to reorganize a unit, then that could be done by editing the unit data files. But it would have to be no finer than in 10ths.

11.2.3.4 Special unit data
See section 11.2.3.1 for the first 6 data fields. Special units have 3 additional data fields:
 Cost to build (synthetic oil units only),
 Column and row for synthetic oil plants that can be placed in only one hex.